1. Field of the Invention
The present invention relates to solid-state image pickup apparatuses. More particularly, the present invention relates to solid-state image pickup apparatuses that include rows and columns of pixels in which, for each column, an analog-to-digital (AD) converter and a digital memory are provided.
2. Description of the Related Art
In response to a recent demand for faster operation of solid-state image pickup devices, complementary metal-oxide semiconductor (CMOS) image sensors have been developed, in which an AD converter is provided for each column of pixels so that analog signals are converted into digital signals for the column, thereby realizing faster operation.
Japanese Patent Laid-Open No. 5-048460 discloses a CMOS image sensor that includes an AD converter for each column of pixels. The sensor is driven in such a manner that AD conversion for each column and output of digital signals to horizontal output lines overlap on the time axis, thereby realizing faster operation. FIGS. 7 and 8 show a schematic circuit diagram of the solid-state image pickup apparatus disclosed in Japanese Patent Laid-Open No. 5-048460 and a timing chart of the same, respectively.
AD converters called cyclic AD converters have also been developed. A cyclic AD converter performs AD conversion by repeating the following: conversion of an input analog signal into a digital signal, calculation of the difference between the input analog signal and a reference voltage to produce a difference signal, application of an appropriate gain to the difference signal, and reinput of the difference signal to a comparator. After the difference signal reinput to the comparator is compared, the difference signal is temporarily held in an internal analog memory included in the cyclic AD converter. Herein, the difference signal held in the internal analog memory is referred to as an internal analog signal.
With respect to a conventional apparatus that includes such AD converters, referring to FIG. 8, an operation for holding analog electric signals and an operation for outputting digital signals from memories holding the digital signals are performed simultaneously. The inventors have found that, in such operations, “streaky noise” artifacts appear in an obtained image. Such streaky noise becomes more apparent particularly when the AD converters have a high resolution and when the output digital signals are amplified. Further, because an image area corresponding to a column with noise is generally brighter or darker than those corresponding to columns without noise, such noise is more recognizable than noise that would occur randomly on an image plane. Human eyes have a tendency to recognize streaky noise more than random noise. Because of such tendency, streaky noise has larger influence on image quality, and should be suppressed.